Wireless mobile communications provides the greatest convenience for users to access voice and data services essentially anywhere and anytime. CDMA communication systems are one of the most promising digital wireless communication systems that can provide the desired mix of voice and data services. In addition, CDMA systems have features such as soft handoff and frequency reuse which makes CDMA a much more attractive choice than other access technologies such as TDMA and GSM.
Soft handoff is a handoff technique that allows a mobile unit to communicate with two or more base stations at the same time. Soft handoff can not only eliminate the potential ping-ponging of handoff but also provides for a smooth transition at handoff without necessitating mute time. Unlike other access technologies, soft handoff can be implemented in CDMA systems because CDMA systems allows the same frequency to be reused between cells. In addition, allowing frequency reuse between cells in CDMA systems means that frequency planning, which is one of the most difficult tasks in other access technologies such as TDMA, GSM, and AMPS etc., is no longer necessary.
However, CDMA systems has its own set of issues and concerns. One issue related to successful soft handoff and allowing frequency reuse between cells everywhere is the interference pattern. In practical field deployment, there are unavoidable situations in some areas that result in the coexistence of several dominant pilots. That is, several base stations has roughly equal path loss to the same area. It should be noted that each base station has a unique pilot which serves as a beacon for that particular base station.
The issues associated with the situation of multiple dominant pilots are three folds. First, since several base stations are of roughly equal path loss, the signals from them are interfering each other. In order to maintain the call, all these base stations have to be involved in the soft handoff with the mobile unit sooner or later. If the mobile unit is moving at a fast relatively speed, then fast handoff is critical to the survival of the call. This fast handoff requirement presents a large challenge for both the mobile units and infrastructure on the processing hardware. Second, when a high number of base stations are involved in a soft handoff this presents a high overhead on the transmit power needs and degrades the actual capacity of the wireless system. Third, the areas which have the multiple dominant pilots are areas that have a higher probability of dropping calls or degrading voice quality.
It would be beneficial to reduce areas that can have multiple dominant pilots in place or equivalently to reduce the number of pilots that are within several dBs of the strongest pilot.